As this is an older model, it's possible it doesn't confirm to any VESA specifications - there are certainly none mentioned in the manual's specifications table.
The optional wall bracket listed as an optional accessory looks fairly proprietary to me and appears to be only available in a standard (flat to the wall) or tilting form and not the corner (cantilever) bracket it sounds like you're after ?
All may not be lost though - it might be possible to use a generic cantilever bracket that would accommodate the TV's mounting hole spacing. You'd need to identify the size of bolts needed and measure the horizontal and vertical distance between the holes, plus try and get your head around whether or not you can use plastic spacers if there are any protrusions and sockets/cables on the rear of the TV that might get in the way of the bracket.
If you do go down this route, it would make sense to ensure that any bracket you buy will also be suitable for a new TV because, at 14 years old, it's probably fair to say that the TV is likely to be approaching the end of it's working life and good quality cantilever brackets aren't particularly cheap